Superheterodyne receiver



April 16, 1935.

w. s. BARDEN SUPERHETERODYNE RECEIVER Filed Oct. 26, 19315 INVENTOR YWILLAM BARDEN ATTORNEY Patented Apr. 16, 1935 Unirse STAT-Es V 1,997,991sUPEnnE'rnnoDrNr; eEoltlvER William Stoddard Bardcn, GrasmereZ lStatenIsland, N. Y.,assignor to 'Radio Corporation of America, a corporationof Delaware r 12 Claims.

lMy. present invention relates to superhetero- 7 95 ing of thelocaloscillator.

While composite rst detector-local oscillator networks of the typedisclosed by me in my copending application Serial No. 592,461, ledFebruary 12th, 1932 function in a satisfactory manduringnorrnalsuperheterodyne reception, there exist certain abnormall situations whenthe funcV` tioning of such networks are poor. `For examkw ple, when areceiver embodying such a network has been tuned to a desired signalfrequency and the cutoff switch of the setehasY been closed, actuationof the switch to open position results in an inoperative condition ofthereceiver. This is particularly noticeable when the frequencyto which thereceiver is tuned corresponds toastrong local station. Againfsuddentuning of such a superheterodynel receiver through a signal frequencyhavingY a strong e'ld intensity causes the aforementioned inoperativecondition. It may also be pointed out that this abnormal. condition mayarise when such a re,- ceiver is employedl in an automobile during al.period when the automobile moves out of an shielding zone, such'as abridge and. permits the signal collector to collect signals of extremelystrong iield intensity. v 'Without resorting to involved. theoreticalcon siderations, it Vis believed sufficient to point out that thesevaforementioned abnormal conditions are usually caused by blocking of thelocal oscillatorlportion of the composite network. A superheterodynereceiver utilizing, for example, a radio frequency ampliiier stage of ahigh or a moderategain v(30 or 40, by way of illustration), apre-selector having a low gain (say l or 2), normal negative bias on thecontrol grid of the radio frequency stage, such normal bias being about-3 volts, and a composite rst detector-local `oscillator network of theaforedescribed type may be rendered inoperative when very strong antennasignals food the composite network. lt may be stated that such strongsignals usually have a magnitude of upwards of 1/4 of a volt. In orderto restore normal operating condition when this abnormality occurs,`itis vnecessary to adjust the tuning device to a frequency differentV fromthe desired signal frequency. Such a requirement necessarily aggravatesa condition which by itself is suiciently confusing to theaveragebroadcast receiver operator. l

Application October 26, l1933,. *SerialNor 695,251

(Cl. Z50-20.).

Accordingly, Iit may be stated that, it is onejof the main objects oflmy rpresent invention toprovide a; 'superheterodyne receiver utilizinga; cornfposite first ydetector-local` oscillatorv'vliereinl` theimpression of vextremely strong" signals "oni the I signal collector`of. the receiver has substantially no effecten the functioning of,the'4 'composite network.

Another important object of the invention may be said to reside in theprovision cfa super',- l0 heterodyne receiver which isfadapted;to'foperate under allv conditions of variationin'the recel ed signalintensity, the essential feature lof the re;- ceiver consisting inimpressing little Vor no'nor'- Amal negativebias on the signal grid ofYthe radio l5 frequency amplier." f, f

Another object of the invention is t0.V pIOvide a method of receivingbroadcast ysignals by the superhe'tero'dyne method, extremelystronglsignais being employed to produce a directcurreiit 2p componentfor reducing the ygain of thej'radio frequency amplifier of thereceiverto a point such that iiooding, ory blocking, of, the local oscillvl'atorof the receiver is prevented, thelofcall os'- c illato-r being embodiediny a compositcnetwpik including a first detector. f

` Still otherobjects .of the invention are to; mprove generally theefficiencyV of superheterodyiie receivers utilizing autodyneVfirst[detectorY networks, and especially to providea receiver ofthistype which is not'only reliable in operation, but which is economicallymanufactured."

The novel features'whichlibelieve to hecharacteristic of my invention.are set. forth in particularity Yinthe appended claims. y"Ilie invenltion itself, however, both as to its organization and' method of'operation will best be understood f by reference to thefollowngde'scriptioi'i,l taken in connection with the drawing, in which .I hereindicated diagrammatioally a circuit vorganizaf 40 tion whereby myinvention may becarried'into effect. u o t Referring now to the.accompanying drawing whichshows in diagrammatic manner a conventionaland well known type/of superheterodyne vi5 receiver, the signalcollector is designated bythe reference numeral I. The signal collectoris the well known grounded antenna circuit, but `itis to .be clearlyunderstood 'that a loop antenna or anV automobile radio receiver signalcollector may l be utilized in place of the grounded antenna circuit.The transformer'Z couplesthe signal collector to the Yinput electrodesof the rstradio frequency amplifier tube 3,' which tube'is preferably ofthe lscreen grid type. The variable v5.5

Lsecond detector I4.

tuning condenser 4 is connected across the secondary 5 of thetransformer 2, and the direct current blocking condenser 6 is interposedbetween the ground side of tuning condenser 4 and the low potential sideof coil 5.

The cathode of tube 3 is connected to ground,

and it is to be clearly understood that it is onev of the essentialfeatures of the present invention that the usual grid biasing networkemployed in the cathode circuit be designed so that little or nonegative bias is imposed on the signal grid of the tube. For this reasonthe usual biasing resistor 1, shunted by the radio frequency by-passcondenser 8, should have a magnitude vsuch that the signal grid of tube3 is only vslightly negative with respect to the cathode. It may even bepreferable in certain situations to dispense with the network 'I and 8,and normally maintain the signal grid at the same direct currentpotential as the cathode.

The anode of tube 3 is connected to a source of positive potential,whichl source is omitted, through a primary of the coupling transformer9. The reference numeral I0 designates the compositel firstdetector-local oscillator which follows the radio frequency amplifier.It is not believed necessary to show the circuit connections of thiscomposite network, since it is fully disclosed and claimed in myaforementioned copending application. It is merely necessary to pointout that the composite network employs a pentode tube, and has as one ofits chief advantages the fact that the signal grid thereof never drawsgrid current. The reference numeral II denotes the variable tuningcondenser of the first detector portion of the network I0, the variabletuning means of the local oscillator portion being omitted to preservesimplicity of disclosure. It is to be clearly understood that thevariable tuning condensers 4 and I I, as well as the tuning condenser ofthe local yoscillator portion which is not shown, are mechanicallyuni-controlled in any well known fashion, the dotted line I2representing such uni-control mechanism.

The intermediate frequency energy output of the network I0 istransmittedto an intermediate frequency amplifier I3, and the amplified output ofthe latter is impressed upon vthe succeeding As is Well known to thoseskilled in the art, the detected output of the network I4 is impressedupon an audio frequency amplifier and finally reproduced in any wellknown manner, as bya loud speaker.

While the'functioning of the present invention is independent of thepresence or absence of an automatic volume control arrangement in thereceiver, such an arrangement has been shown to illustrate. t-he pointthat even the presence of sucha gain control arrangement is of littlevaluev in solving the problem which has heretofore been discussed.

The automatic volume control arrangement shown in the drawing is of apurelyconventional type and is merely shown by way of illustrating oneform of the arrangement. Since the gain of the radio frequency amplifier3 is to be varied in response to variations in received carrieramplitude, a device, such as a diode rectifier I5, is utilized to derivea direct current control potential from received carrier energy. Thediode anode is customarily connected to one sideof the input circuit ofthe second detector I4 through a signal transmission condenser I6, whilethe cathode of the diode is grounded so as to provide a signal returnpath to the grounded side of the 'merals I9, 20.

detector input circuit. Between the diode electrodes there is usuallyconnected an impedance I1, which may be a resistor, and a suitableselected point of negative direct current potential of the impedance isconnected by a direct current connection I8 to the grid circuit of theradio frequency amplifier. This direct current connection has also beendesignated AVC" and the usual radio frequency filter network of theautomatic volume control path is designated by the nu- Of course, gaincontrol connections may be made to the network I0 and the intermediatefrequency amplifier network I3, but these are omitted since they arewell known to those skilled in the art.

As stated heretofore, whereas it is usual to employ a negative biasabout -3 Volts on the signal grid of the radio frequency amplifier, inthe present case there is only a slight negative bias thereon, as forexample not over -1/2 volt. Because of this potential relation strongsignals at the signal collector I, in resonance with the tuned circuit,are better'able to cause a flow of grid current in the grid circuit ofthe radio frequency v amplifier. This grid current fiows through a dioderectifier circuit wherein the diode comprises the grid and cathode ofthe amplifier tube 3. The

diode circuit may be traced from the grid of tube 3 through the coil 5,through the resistor I9, the lead I8, the resistor Il, ground, to theground of the cathode of tube 3, and the cathode of the latter. Thediode action causes a rnegative grid bias to be developed acrossresistor I9 and whatever other-resistance is included in this diodecircuit. This negative bias decreases the gain of the radio frequencyamplifier stage. and tends to prevent flooding of the composite networkI0.

Itl will, therefore. be seen that when the tuning condensers 4 and I Iare adjusted to receive a signal frequency having a high fieldintensity, up-

wards of 1A of a volt for example, and such strong signals are suddenlyimpressed upon the receiver, the gain of the radio frequency amplifierwill be reduced by the positive signal swing sufiiciently to preventblocking of the local oscillator portion of the composite network I0. Aspointed out above, such sudden impression of strong signals occurs whenswitching the receiver to operative condition while tuned to thefrequency of the strongsignals, suddenly tuning through such a strongsignal frequency, or suddenly moving the receiver into the strong field.In any of these cases the grid of the amplifier tube 3 is swungpositive, and the resultant flow of grid current through the diodecircuit including the grid and cathode of tube 3 creates a negative gridbias potential which is sufiicientto reduce the gain of tube 3 below theoscillator blocking point.

In designing the receiver embodying the present invention it isdesirable to have a high stepup ratio between lthe antenna .circuit Iand the signal grid circuit of the radio frequency amplifier in order toobtain the diode action without having to make the antenna signalexcessively strong. For a given and prerequisite amount of radiofrequency gain from the antenna. to the gridof the first detector it isin the best interests of the diode action to have a great amount of thatgain ahead of the radio frequency grid, and a smallamount of that gainin the radio frequency amplifier stage. The high step-up ratio betweenthe antenna and the first radio frequency amplifier, and thecorresponding reduction in the required radio frequency gain in theradio frequency stage itself are favorable to the maintenance of a lownegative bias on the grid of the radio frequency amplifier tube. In thisway the erratic performance of the receiver which has been describedheretofore is effectively prevented.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described, but that many modications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims,

What I claim isz- 1. A method of receiving signal energy of a selectedfrequency which consists in amplifying collected signals of a selectedfrequency, impressing the amplified signals upon a composite firstdetector-local oscillator, utilizing the resultant intermediatefrequency energy, deriving prior to said impression from extremelystrong signals of said frequency a direct current component, anddecreasing the amplification of said collected signals when saidextremely strong signals are suddenly collected.

2. In a superheterodyne receiver, a tunable radio frequency amplifier,an autodyne first detector, means for utilizing the output of saiddetector, the signal grid of said amplifier being slightly negative withrespect to the cathode thereof, and means connected between the signalgrid and cathode of said amplifier for developing a grid biasing directcurrent potential from extremely strong signals suddenly impressed uponsaid amplifier.

3. In a superheterodyne receiver, a tunable radio frequency amplifier,an autodyne first detector, means for utilizing the output of saiddetector, the signal grid of said amplifier being slightly negative withrespect to the cathode thereof, and means including a direct currentconnection connected between the signal grid. and cathode of saidamplifier for developing a grid biasing direct current potential fromextremely strong signals suddenly impressed upon said amplifier.

4. In a superheterodyne receiver, a tunable radio frequency amplifier,an autodyne first detector, means for utilizing the output of saiddetector, the signal grid of said amplifier being slightly negative withrespect to the cathode thereof, and means including an impedanceconnected between the signal grid and cathode of said amplifier fordeveloping a grid biasing direct current potential from extremely strongsignals suddenly impressed upon said amplifier.

5. In a superheterodyne receiver, a tunable radio frequency amplifier,an autodyne first detector, means for utilizing the output of saiddetector, the signal grid of said amplifier being slightly negative withrespect to the cathode thereof, and means including a resistive pathconnected between the signal grid and cathode of said amplifier fordeveloping a grid biasing direct current potential from extremely strongsignals suddenly impressed upon said amplifier.

6. In a superheterodyne receiver, a tunable radio frequency amplifier,an autodyne first detector, means for utilizing the output of saiddetector, the signal grid of said amplifier being slightly negative withrespect tothe cathode thereof, means connected between the signal gridand cathode of said amplifier for developing a direct current potentialfrom extremely strong signals suddenly impressed upon said amplifier,and additional means including said developing means as a part thereoffor automatically regulating the gain of said amplifier in response tovariations in received carrier amplitude.

'7. In a superheterodyne receiver, a radio frequency amplifier having atunable input circuit including a variable tuning condenser, a compositefirst detector-local oscillator having a signal input circuit includinga variable tuning condenser coupled to the output circuit of saidamplifier, yan intermediate frequency amplifier coupled to theintermediate frequency output circuit of said composite network, andmeans for utilizing the amplified intermediate frequency energy, adirect current path between the signal grid and cathode of said radiofrequency amplifier, said path including a resistive impedance, thesignal grid of said radio frequency amplifier normally being maintainedslightly negative with respect to the cathode, said resistive impedancedeveloping a direct current potential thereacross of a magnitudesufficient to decrease the gain of said radio frequency amplifier to apoint such that blocking of the local oscillator of said compositevnetworkis prevented when extremely strong signals of the frequency towhich said variable tuning condensers are adjusted is impressed on theinput circuit of said radio frequency amplifier.

8. In a receiver of the type defined in claim 7, an automatic volumecontrol arrangement including said resistive impedance as a part thereofconnected between the output of said intermediate frequency amplifierand the signal grid circuit of said radio frequency amplifier.

9. An improvement in a superheterodyne receiver of the type including acomposite local oscillator-first detector circuit preceded by a tunableradio frequency amplifier, and the local oscillator network of thecomposite circuit tending to become inoperative when strong signals of aselected frequency are suddenly impressed on said amplifier, saidimprovement comprising means for maintaining the signal grid of thevamplifier at a direct current potential which at no time becomespositive with respect to the amplifier cathode potential, a directcurrent path between the amplifier signal grid and cathode including animpedance, said impedance having a value sufiicient to develop a directcurrent voltage for negatively biasing said signal grid to reduce thevamplifier gain to a level for preventing said inoperativeness when saidstrong signals are received.

10. In a receiver of the type defined in claim 9,

a signal collector, and a signal voltage step-up device between thecollector and amplifier.

11. In a receiver as defined in claim 9, said maintaining meanscomprising an impedance in the space current path of the amplifieradapted to bias the signal grid negatively.

12,. In areceiver as defined in claim 9, an automatic volume controlcircuit connected between the composite circuit output and the signalgrid of the amplifier and including the said path as a part thereof.

WILLIAM STODDARD BARDEN.

